Axial flow rotary machines, such as gas turbine engines used for the propulsion of aircraft, have rotor assemblies disposed within the engine. Each rotor assembly is used to transfer energy between the rotor assembly and gases flowed through the engine. The rotor assembly is commonly provided with rotor disks each having a plurality of rotor blades.
Each rotor blade typically has a root which adapts the blade to engage a corresponding groove in the disk for retention of the rotor blade. An airfoil extends outwardly from the root and has a convex side and a concave side for interacting with the working medium gases. The rotor blade may also be provided with a shroud, such as a tip shroud at the outer end of the airfoil. Each shroud has laterally (circumferentially) extending faces and axial extending faces. An example of an axially extending face is the notch face. The shroud extends circumferentially to engage the shroud of each of the adjacent rotor blades at the notch face.
During operation of the machine, the rotor blades in the turbine section extend outwardly across the working medium flow path. The rotor blades receive energy from the working medium flow path to drive the rotor assembly about its axis of rotation. In the compression section, the rotor blades transfer energy to the gases to compress the gases as the rotor blades are driven about the axis of rotation by the rotor assembly. As a result, the rotor blades and shrouds are subjected to fluctuating gas flows and high temperatures.
These fluctuating gas flows induce vibrations in the blades which are damped by sliding friction as the shrouds rub against each other. Such rubbing causes wear. The notch face of the shroud at the point of contact has a hard facing material to extend the life of the rubbing surfaces on new blades and is replaced on used blades after the notch becomes worn.
The present invention is concerned with a fixture and method for disposing shroud material, such as a weld material for hard facing a cross notch, on the substrate of a used rotor blade or on a new, rotor blade which is partially formed. Accordingly, the term "rotor blade" includes a partially formed rotor blade having a substrate that adapts the rotor blade shroud to receive such material, or is a used rotor blade having a portion of the shroud surface machined away to form a substrate prior to restoration of the rotor blade.
One method of providing hard facing material to a shroud is to melt hard face material at the notch face by passing an electrical current through the material, such as by welding. This results in a pool of molten material. The molten material is used to build up the thickness of material on the notch face as it hardens. Unfortunately, the pool of molten material coats the top of the shroud, coats the flow path surface beneath the shroud and extends radially past the notch face surface as it hardens.
It is relatively straight forward to remove the deposited material from the top of the shroud because this is not a flow path surface and is easily accessible during the rotor blade fabrication process. The underside of the shroud is a different matter because it includes hard face material protruding from the substrate and attached to the shroud. The protrusion adds weight to the shroud at a critical location and disrupts the aerodynamic smoothness of the flow path surface. The once molten material, now hardened into a lump of hard face material, is removed from the under side of the shroud to decrease rotational stresses in the shroud to acceptable levels and to form a surface having an acceptable aerodynamic smoothness.
One approach is to hand blend the weld material such as by use of a hand held belt sander or hand held surface grinder. Such work is time consuming, ergonomically undesirable and expensive. Machining is not an attractive alternative because of difficulty in locating a partially formed part, such as a rotor blade during manufacture, that has a substrate which receives the molten material. The typical method is to rigidly support the rotor blade for the machining operation by engaging the partially formed root of the rotor blade. A rotor blade with an unfinished roots is difficult to locate using the root as a datum for the machining operation because the location of the shroud then has much variability in a locating fixture. Using a cast block datum around the airfoil is an alternative, but the block prevents access to the underside of the shroud and prevents removing material from the flow path surface located there.
Accordingly, scientists and engineers working under the direction of Applicants' assignee, have sought to develop a method and an apparatus for use with the shrouds rotor blades which decreases the difficulties associated with removing hard face material from the flow path surface of the rotor blade shroud.